While there is no specific tag for 'reimbursement from friend', there are ways to deal with a transaction so it doesn't show as a higher spend than you actually made. We are planning some improvements to the tagging system to better handle expenses and reimbursements, but in the meantime, the following workaround scenario may be helpful.
You share a gas bill for £100 with your flatmate. Your share of the cost is £50, but you pay the full £100 to the gas company and your flatmate reimburses you for the other £50. If the payment is tagged as Household > Gas then it will appear that you have spent £100 on gas this month rather than £50.
To rectify this you will first need to use the transaction Split function. Take the £100 transaction and split it into two transactions for £50 each. Leave the original parent transaction's description and tag as it is (eg, 'British Gas', Household > Gas). On the new £50 transaction you can choose to amend the description (eg 'British Gas - flatmate's share') and then change the tag to one from the Transfers group that you are unlikely to use for anything else, eg Transfer to other account > Investment - Other. For the corresponding incoming £50 transaction from your flatmate you will then also need to tag this as a transfer, eg - Transfer from other account > Investment - Other.
So why am I applying these?
The reason for applying the Transfer tags to the transactions that don't represent your share of the bill is because it makes it easier to hide them in the reporting. Obviously all transactions will still appear on your main transaction history, and if the transactions were made on a current account you will also see all of them in the In:Out reporting view. But if you use the Income: Spend reporting view this hides transactions that are tagged as transfers or credit repayments. In this instance you will then only see your £50 share of the Gas Bill, but not the other £50 of the bill or your flatmate's contribution. You can also toggle the settings on the Outgoings by Group dashboard chart to remove the transfers from the chart and again only show your £50 share of the bill.
Find out more about splitting transactions